Java thread state and switch

Irving the procedural ape 2020-11-11 10:10:17
java thread state switch


Java Thread state and switch

One 、 What is? Java Thread state

stay Java In the program , Used to describe Java Six states of thread :

  • newly build (NEW): Current thread , It's just been built , Not yet started .
  • function (RUNNABLE): Current thread , In competition CPU Time sliced or acquired CPU The state of the time slice .
  • wait for (WAITTING): Current thread , In dormancy , No participation CPU The state of time slice competition .
  • Wait regularly (TIMED_WAITTING): Current thread , In a timed sleep , Don't take part in CPU The state of time slice competition .
  • Blocking (BLOCKED): Current thread , It's in a jam , No participation CPU The state of time slice competition .
  • End (TERMINATED): Current thread , In a state of final cessation .

New state , Can only enter the running state . The termination state cannot be changed to any other state .

wait for / Timed waiting and blocking , The difference is that the latter requires an event signal ( If other threads abandon the exclusive lock required by the current thread ), Before you can switch state . Of course , It's OK to force closure .

Java Thread implementation is not subject to JVM Normative constraints , So the implementation of different virtual machines , It's often different . At present, the mainstream HotSpot It's going to be every Java Threads are mapped directly to native threads of an operating system , Thus, the operating system completes a series of thread scheduling

Two 、 Where to see Java Thread state

see Java Thread state , There are three main ways :

  • java.lang.Thread.State You can directly see Java Six thread states of
  • Java Runtime , Inside the program, you can use Thread.getState() Get target thread status
  • Java Runtime , Outside the program, you can use jstack Tools such as , View thread status

of jstack Wait for tools to use , There will be blogs later , To elaborate on .

3、 ... and 、 When to change Java Thread state

Java Thread state switching . No verbosity , Directly above .
 Insert picture description here
This picture covers Java Various methods of thread state switching . Compared with some pictures on the Internet , More detailed .
If something is missing , Can tell me , I'll fill it in time .

Four 、 Who is using Java Thread state

Daily development , We don't interact directly with thread state .
We often use JDK Packaged tools , Such as JUC All kinds of tools under the bag .

Take a chestnut

Application in thread pool

Location :com.sun.corba.se.impl.orbutil.threadpool.ThreadPoolImpl#close

 // Note that this method should not return until AFTER all threads have died. public void close() throws IOException { // Copy to avoid concurrent modification problems. List<WorkerThread> copy = null; synchronized (workersLock) { copy = new ArrayList<>(workers); } for (WorkerThread wt : copy) { wt.close(); while (wt.getState() != Thread.State.TERMINATED) { try { wt.join(); } catch (InterruptedException exc) { wrapper.interruptedJoinCallWhileClosingThreadPool(exc, wt, this); } } } threadGroup = null; }

Actually check JDK After the discovery ,JDK In fact, there are not so many examples of spicy , And most of them are directly related to the thread state , It's also something about status checking .
This is said at the beginning of the article ,Java Thread operation , It's very low-level , Even the implementation is not included in the virtual machine specification .
Mainstream HotSpot, It's also a direct way to Java Threads map to system threads , By the system to carry out a series of thread scheduling processing .
therefore , stay JDK in , There is little direct processing of the state of the thread .

5、 ... and 、 Why thread state is needed

1. Why the concept of thread state is needed

This problem , It can be explained from two aspects : Life cycle and resource management

  • One side , Thread state is a good description of the whole life cycle of a thread , The different stages in the life cycle are divided effectively .
  • On the other hand , Resources are limited , The demand is infinite . So we need to schedule the system resources consciously , Make rational use of comparative advantages , The pursuit of Pareto optimality .

Realize the latter , It is the use of thread in the different stages of the life cycle of this natural attribute of the description of state , The grouping that was carried out .CPU Scheduling only needs to focus on the running state of the thread . And jam , Wait for threads , They all have their own way of handling . Finally get the resources ( Development response to complexity , The runtime consumes system resources , The growth of the user's mental model, etc ) The optimal allocation of .

2.JDK Why do we need to define Java Thread state

I said before ,Java Thread state is rarely used directly in . So why are you still in JDK In the definition of Java The six thread states of ?
One side , Through information transparency , Reduce the cost of building mental models for users . Such as , Now we can print logs , Breaking point , Quickly understand Java State of each thread of , And have a clear understanding of the causes . This greatly improves our understanding of Java Understanding of threads , And embrace more happily JUC Package, such as thread pool and other tools .
On the other hand , By enumerating states that can be applied directly , We can make a good secondary development of the existing tools . For example, we can expand AQS, And add the verification of thread state in it , So we can get a customized thread synchronization tool .

6、 ... and 、 How to use thread state

How to use it , I've already said that : Study Java Threads , Custom thread related tool development .
Here is an example of thread learning demo:

/** * @program: learning * @description: Used to confirm thread status problems * @author: Jarry * @create: 2020-10-26 22:25 **/public class ThreadState { public static void main(String[] args) { threadStateTest();// threadStateTest2();// threadStateWithBlocked();// threadStateWithException();// threadStateWithSuspend(); } /** * Practice has proved :Thread.suspend() And Thread.resume() Does not change the thread state * The thread state should be Waiting, Just Waiting. The Timed_Waiting Just Timed_Waiting * Thread.suspend() And Thread.resume() Just suspend the target thread ( And does not release lock resources ) */ private static void threadStateWithSuspend() { Thread thread1 = new Thread(() -> {// LockSupport.park(); LockSupport.parkNanos(2000000000); }); thread1.start(); printThreadState(thread1); LockSupport.parkNanos(500000000); printThreadState(thread1); thread1.suspend(); printThreadState(thread1); LockSupport.parkNanos(500000000); printThreadState(thread1); thread1.resume(); LockSupport.parkNanos(500000000); printThreadState(thread1);// LockSupport.unpark(thread1); } /** * Show thread blocking status */ private static void threadStateWithBlocked() { Runnable runnable = new Runnable() { @Override public void run() { synchronized (ThreadState.class) {// LockSupport.parkNanos(2000000000); LockSupport.park(); } } }; Thread thread1 = new Thread(runnable); Thread thread2 = new Thread(runnable); thread1.start(); LockSupport.parkNanos(500000000); thread2.start(); // Add the following intervals , Then the result :Runnable->Blocked // inference :Thread.start() The thread state is set to Runnable, And then I met sync Lock of , Then switch to Blocked state // LockSupport.parkNanos(500000000); printThreadState(thread2); LockSupport.parkNanos(500000000); printThreadState(thread2); LockSupport.parkNanos(500000000); LockSupport.unpark(thread1); LockSupport.unpark(thread2); } /** * from .........
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